At a very high level:
1. In working storage Add DCLGEN for tables and remove IO areas.
2. Linkage section - No need of PCB masks
3. Procedure division - Remove using or Entry statement which ever defined
4. Instead of CBLTDLI calls code equivalent of this in DB2 statements. Converting IMS calls to DB2 equivalent is tricky or little complicated, bcos IMS is hierachical and DB2 is relational. You can do simpler using Db2 if you know exactly what business logic the IMS calls are used within program.
5. Create PLANS, delete PSBs
6. Also you need to consider if IMS program is BMP, Checkpoint restart etc.. in DB2

This is not complete list, once you start migrating you can come up with better steps.